20.—(1) For the purposes of sections 18 and 19—

(a) a person uses force in relation to another person or property not only when he or she applies force to, but also where he or she causes an impact on, the body of that person or that property;

(b) a person shall be treated as using force in relation to another person if—

(i) he or she threatens that person with its use, or

(ii) he or she detains that person without actually using it; and

(c) a person shall be treated as using force in relation to property if he or she threatens a person with its use in relation to property.

(2) Sections 18 and 19 shall apply in relation to acts immediately preparatory to the use of force as they apply in relation to acts in which force is used.

(3) A threat of force may be reasonable although the actual use of force may not be.

(4) The fact that a person had an opportunity to retreat before using force shall be taken into account, in conjunction with other relevant evidence, in determining whether the use of force was reasonable.